A column can be assigned a default value. When a new row is
created and no values are specified for some of the columns,
those columns will be filled with their respective default
values. A data manipulation command can also request explicitly
that a column be set to its default value, without having to know
what that value is. (Details about data manipulation commands are
in Chapter 6.)

If no default value is declared explicitly, the default value
is the null value. This usually makes sense because a null value
can be considered to represent unknown data.

In a table definition, default values are listed after the
column data type. For example:

The default value can be an expression, which will be
evaluated whenever the default value is inserted (not when the table is created). A
common example is for a timestamp column to
have a default of CURRENT_TIMESTAMP, so
that it gets set to the time of row insertion. Another common
example is generating a "serial
number" for each row. In PostgreSQL this is typically done by
something like: